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584

Expansion and Temperatiire.

heated : the fire, near the floor, rarefies the immediately sur-
rounding air, which rises to the ceiling and falls again when
cooled against the walls. Water, being a bad conductor, cannot
well be heated by any but the convection method, hence the
adoption of ~a low position, in a boiler, for the fire-grate.

Expansion is the result of the application of heat to all
bodies, whether solid, liquid, or gaseous; the first being least
and the last most expansible. Many examples may be suggested
of the application of this law, some useful and some detrimental.
Shrinking of gun coils is of the former type, while the endlong
clearance between rail lengths of the permanent way avoids the
injurious effects of the summer heat. Fig. 327 shews how work
might be done by the expansion of solids. Water, between
32░ and 39'i░ F., is an exception to the law of expansion; during
that period it contracts as the temperature increases. Cast iron
also expands when solidifying in the mould, and bismuth and
antimony follow the same rule; gold, silver, and copper contract.

Measurement of Heat.ŚWe proceed to measure intensity
and quantity of heat, bearing in mind, however, that heat is not a
substance but a form of energy.                                          !

Temperature is a measure of the intensity of heat, being
registered on a thermometer or pyrometer. Thermometer^ are
based on the expansion of liquids or gases in a glass bulb, which
then rise in a capillary stem from which air has been exhausted.
Mercury or alcohol are the usual liquids, the former for ordinary
and comparatively high temperatures, and the latter for very low
temperatures : the boiling point of mercury being very high, and
the freezing point of alcohol very low. The freezing and boiling
points of water, under atmospheric pressure, being unchangeable,
are first marked on all thermometers, after which the graduations
are spaced according to one t>f the following methods :

Thermometer.
	Divisions between Freezing and Boiling.
	Freezing Point
	!
 Boiling Point,  j

Fahrenheit ... Centigrade ... Reaumur   ...
	1 80
 100
 80
	^
 oc
	21.2░ 100*
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